As a power semiconductor device, a device is desirable that is operable at a high voltage close to a theoretical breakdown voltage of semiconductor. Therefore, if an integrated circuit controls an external system which uses a high voltage, the integrated circuit requires a semiconductor device built therein for controlling the high voltage, and the high voltage semiconductor device is required to have a structure which has a high breakdown voltage. Meaning, in a case of a drain or a source of a transistor having the high voltage applied thereto, it is required that a punch through voltage between the drain and source and a semiconductor substrate and the breakdown voltage between the drain and source and a well or the substrate is higher than the high voltage applied thereto.
Of the high voltage semiconductor devices, since an Lateral Diffused MOS (LDMOS) which is an MOS for the high voltage has a channel region and a drain electrode isolated by a drift region disposed therebetween, and controlled by a gate electrode, the LDMOS has a suitable structure for the high voltage.
FIG. 1 illustrates a section of an LDMOS transistor.
As illustrated in FIG. 1, the LDMOS transistor is provided with high voltage N well 110, device isolation film 120, P type body 115, gate oxide film 130, first source region 140 and second source region 150, drain region 145, gate polysilicon 160 and spacers 165. Device isolation film 120, which is formed under gate polysilicon 160 of the LDMOS transistor, sustains an appropriate breakdown voltage BV. However, since current flows around device isolation film 120 to drain region 145, making a current path longer, a turn on resistance Ron characteristic becomes poor.